Pump Element for a Hydraulic Unit having a Pump Piston

ABSTRACT

A pump element for a hydraulic unit of a vehicle braking system includes a pump piston that has two piston parts. A first piston part of the two piston parts is produced from a sintering material.

PRIOR ART

The invention relates to a pump element for a hydraulic unit of a vehicle brake system, having a pump piston which has two piston parts. The invention relates, furthermore, to a method for producing such a pump piston and to the use of a pump element of this type.

Generic pump elements are used in hydraulic units of vehicle brake systems, in order, particularly during the regulation of brake pressure, to generate brake pressure actively or reduce brake pressure actively on associated vehicle brakes. For this purpose, as a rule, the pump elements have at least two pump piston sides which, in the configuration of an axial piston pump, are driven by an eccentric. The eccentric engages on one end face of the respective pump piston, in order to move it axially back and forth for a pumping movement. This end face of the pump piston is consequently exposed to particular mechanical load. The opposite end face of the pump piston serves for sealing off a pump space or pressure space of an associated pump cylinder. For this purpose, this end face of the pump piston is sealed off with respect to the pump cylinder by means of a sealing element arranged on the pump piston.

DISCLOSURE OF THE INVENTION

According to the invention, a pump element for a hydraulic unit of a vehicle brake system, having a pump piston which has two piston parts, is provided. Of the two piston parts, a first piston part is produced by means of a sintered material or sintered stock. For the piston part according to the invention, the sintered material used is especially preferably Al₂O₃.

Sintered material is produced in that one or more fine-grained or pulverulent, in particular metallic substances are heated, mostly under increased pressure, to temperatures below their melting temperature. In contrast to a pure melt, however, in this case none or at least not all of the starting materials are melted down. Hence, formulated in colloquial language, the starting materials are “caked together”. Sintering is therefore a primary forming method.

During sintering, the starting materials are brought, if appropriate by means of a binder, into the form of a desired workpiece. This takes place, as a rule, by pressing. This green compact, as it is known, is subsequently compressed and cured by heat treatment below the melting temperature. In sintering, casting is also employed as shaping method in addition to pressing.

In general, sintered parts have a comparatively porous configuration in terms of their material structure and therefore, in view of requirements, such as seal tightness and compressive strength, are not advantageous for use as pump pistons. According to the invention, however, such a sintered part is provided in combination with a further piston part. As a result, each of the two piston parts can be assigned specific different functionalities. In this case, the sintered part according to the invention can be given the functions advantageous for it, and therefore the other advantages of such sintered parts can be utilized at the same time. Thus, for example in comparison with milled parts, sintered parts have considerably lower manufacturing costs. In light of their tensile strength which is in this case achieved, they are far better than equally costly components made from other materials.

Particularly advantageous in this regard is the use of sintered steel for the piston part according to the invention, and therefore this can also be designated as a steel sintered part. A component of this type, while having identical or lower costs, tolerates higher load, and therefore, in particular, it becomes possible to operate the associated pump piston at higher pressures.

These advantages according to the invention are especially beneficial in that the sintered part according to the invention is assigned the function of a valve seal, a valve seat being formed around a valve orifice on the sintered part. The sintered material selected according to the invention results at the valve seat in an advantageous optimum of low production costs and low wear. The valve seat of this type made from sintered material has a long service life, while at the same time being produced cost-effectively.

In an advantageous development of the pump element according to the invention, the first piston part is configured with a ring shape, in which at least one duct passing radially through the ring shape and having an axially open configuration is provided. The duct of this type serves particularly as an inflow line for brake fluid radially from the outside radially inward into the interior of the pump piston according to the invention. The duct is advantageously formed between the two piston parts according to the invention in such a way that it can be formed without extra outlay at the same time the sintered part is being produced. Separate machining with through bores, as is the case at the present time with regard to milled or drilled piston parts, is therefore dispensed with.

In this case, especially preferably, the at least one duct is configured with duct walls obliquely oriented axially. The geometry of the duct and, in particular, the entire sintered part according to the invention is then configured in such a way that this sintered part can be formed by means of two die halves of a sintering mold, and in this case the two die halves merely need to be moved in the axial direction. The axially oblique duct walls form mold removal slopes, by means of which the sintered part can be removed from the sintering mold after production. In particular, the sintered part according to the invention preferably has no undercuts, such as are to be regularly found in known valve seat components of pump pistons of a hydraulic unit of a vehicle brake system.

On the first piston part, especially preferably, the valve orifice is designed with a valve seat, the surface of which is impregnated. By means of impregnation, both the sealing-off and the wear situation on the valve seat can be improved.

Preferably, for this purpose, the entire sintered part is impregnated. Alternatively, hardening or stamping of the valve seat, heated for this purpose to annealing temperature, is expedient. For annealing, the valve seat sintered part according to the invention is preferably heated by means of an induction current device or a laser device. Furthermore, by means of steam treatment, the pores of the sintered material can be closed and the wear resistance of the valve seat can be increased.

Advantageously, furthermore, the first piston part partially surrounds the second piston part. By means of such a configuration, it is possible to have an especially large inlet cross-sectional area for the at least one said duct for the inlet of brake fluid into the interior of the pump piston. The configuration, surrounding the second piston part, of the first piston part then makes it possible for the first piston part to be joined, in particular pressed, onto the outside diameter of the second piston part. Consequently, the entire outer circumference of the first piston part, which in this case is even larger than the outer circumference of the second piston part, is available for the inlet cross-sectional area.

In order to obtain a pump element which overall can be produced very cost-effectively, all the essential delivery and sealing-off functions can be integrated in the first piston part which according to the invention is configured as a sintered part. In particular, the second piston part may have a circular-cylindrical configuration. By means of this circular-cylindrical shape, this second piston part can be produced very cost-effectively, so that even a comparatively costly material can be selected for this.

Thus, in the pump element according to the invention, the second piston part is advantageously to be produced from ceramic. A piston part produced in this way from ceramic has high wear resistance. At the same time, it is comparatively lightweight and therefore also has especially low mass inertia. As a cost-effective alternative, the second piston part according to the invention may be formed by plastic or by steel.

For the pump element according to the invention to have a design which overall is simple to assemble, it is advantageous to arrange on the first piston part a valve cage which, in particular, is formed in one piece with a sealing ring. The sealing ring may in this case be configured as a high-pressure sealing ring and be pressed onto the first piston part. Both the valve cage and the sealing ring can be produced cost-effectively in one operation by injection molding with a correspondingly suitable plastic. For simple assembly, preferably when the valve cage, together with the sealing ring, is being mounted, an associated valve spring and a valve closing body are inserted at the same time.

The invention is also directed, correspondingly to the above explanation, to a method for producing a pump piston of a pump element of a hydraulic unit of a vehicle brake system, in which a first piston part is produced by a sintering method and is pressed onto a second piston part.

Such a pump element according to the invention is preferably used on a hydraulic unit of a vehicle brake system. By means of the solution according to the invention, the requirements with regard to reliable pressure sealing can be fulfilled especially well there throughout the entire service life, and at the same time along with especially low production and assembly costs.

An exemplary embodiment of the solution according to the invention is explained in more detail below by means of the accompanying diagrammatic drawings in which:

FIG. 1 shows a side view of an exemplary embodiment of a pump piston according to the invention with a first and a second piston part,

FIG. 2 shows a longitudinal section of the pump piston according to FIG. 1,

FIG. 3 shows a perspective view of the first piston part of the pump piston according to FIGS. 1, and

FIG. 4 shows a longitudinal section of the first piston part according to FIG. 3.

FIG. 1 illustrates a pump piston 10 of a pump element, not illustrated any further, of a hydraulic unit of a vehicle brake system. The pump piston 10 is configured in two parts and has a first annulus-shaped piston part 12 and a second circular-cylinder-shaped or circular-cylindrical piston part 14. Furthermore, a sealing ring 16 and a valve cage 18 are located on the pump piston 10.

The sealing ring 16 serves for sealing off the pump piston 10 on the inside at a pump cylinder, not illustrated. The pump piston 10 is mounted displaceably in this pump cylinder by means of the sealing ring 16, in order in an associated pump space or pressure space to exert high pressure upon a brake fluid located there. The brake fluid passes into this pump space through an inlet valve, of which the valve cage 18 forms an integral part. The inlet valve includes, furthermore, a valve orifice 20 which is formed centrally in the first piston part 12. A valve seat 22 is formed around the valve orifice 20 on the end face of the first piston part 12 and likewise belongs to the inlet valve. A valve closing body is urged by means of a valve spring (neither is illustrated) against the valve seat 22, by means of which valve closing body the valve orifice 20 can then be selectively closed or opened.

Located on the outer portion of the associated end face of the first piston part 12 is a shoulder 24, against which the sealing ring 16 is pressed, the sealing seat 16 being produced in one piece with the valve cage 18 from plastic by means of injection molding. The valve cage 18 is thereby also held, fixed in place, on the first piston part 12.

Shaped out on the first piston part 12, on the opposite end face of the latter are three end-face ducts 26 passing radially through the annular shape of the first piston part 12. The ducts 26 serve for feeding brake fluid radially from the outside radially inward to said inlet valve and then further on into the associated pressure space.

The ducts 26 are also configured as being open in the axial direction, and they are equipped with axially slightly oblique duct walls 28. Overall, therefore, a crown shape is obtained on this end face of the first piston part 12. The duct walls 28 oblique in this way serve as mold removal slopes which, inter alia, make it possible that the first piston part 12 can be formed from a sintered material, in particular as a steel sintered component.

To produce appropriate surface quality, particularly at the valve seat 22, this steel sintered component has been impregnated overall after its production as a sintered part.

In the region of the crown shape of the first piston part 12, an essentially right-angled step 30 is formed radially on the inside on those portions of the piston part 12 which are located between the ducts 26. By means of this step 30, the adjacent second piston part 14 is surrounded on its end face 32 there, over its entire outer circumference 34, by the first piston part 12. The surround is in this case configured both positively and nonpositively, in that the first piston part 12 has been pressed axially onto the second piston part 14.

As already mentioned above, the second piston part 14 has itself a circular-cylindrical configuration and in this case is produced from a high-strength and at the same time very lightweight ceramic material. In this case, when the associated pump element is in operation, the end face 36, lying opposite the end face 32, of the second piston part 14 serves for pushing the pump piston 10 by means of an eccentric, not illustrated, which rotates there. 

1. A pump element for a hydraulic unit of a vehicle brake system, comprising: a pump piston having two piston parts, wherein a first piston part of the two piston parts includes a sintered material.
 2. The pump element as claimed in claim 1, wherein the first piston part is configured with a ring shape and includes at least one duct passing radially through the ring shape and having an axially open configuration.
 3. The pump element as claimed in claim 2, wherein the at least one duct has duct walls axially oriented obliquely.
 4. The pump element as claimed in claim 1, wherein the first piston part defines a valve orifice with a valve seat, the valve seat having a surface that is impregnated.
 5. The pump element as claimed in claim 1, wherein the first piston part is configured to partially surround the second piston part.
 6. The pump element as claimed in claim 1, wherein the second piston part is of circular-cylindrical configuration.
 7. The pump element as claimed in claim 1, wherein the second piston part includes ceramic.
 8. The pump element as claimed in claim 1, further comprising a valve cage arranged on the first piston part, the valve cage being configured in one piece with a sealing ring.
 9. A method for producing a pump piston of a pump element of a hydraulic unit of a vehicle brake system, comprising: forming a first piston part by a sintering method; and pressing the first piston part onto a second piston part.
 10. A hydraulic unit of a vehicle brake system, comprising: a pump element for the hydraulic unit, the pump element including a pump piston having two piston parts, wherein a first piston part of the two piston parts includes a sintered material. 